Abstract
What is the topic of this review? Biological and methodological factors associated with the variable changes in cardiorespiratory fitness in response to endurance training. What advances does it highlight? Several biological and methodological factors exist that each contribute, to a given extent, to response variability. Notably, prescribing exercise intensity relative to physiological thresholds reportedly increases cardiorespiratory fitness response rates compared to when prescribed relative to maximum physiological values. As threshold-based approaches elicit more homogeneous acute physiological responses among individuals, when repeated over time, these uniform responses may manifest as more homogeneous chronic adaptations thereby reducing response variability. Changes in cardiorespiratory fitness (CRF) in response to endurance training (ET) exhibit large variations, possibly due to a multitude of biological and methodological factors. It is acknowledged that ∼20% of individuals may not achieve meaningful increases in CRF in response to ET. Genetics, the most potent biological contributor, has been shown to explain ∼50% of response variability, whilst age, sex and baseline CRF appear to explain a smaller proportion. Methodological factors represent the characteristics of the ET itself, including the type, volume and intensity of exercise, as well as the method used to prescribe and control exercise intensity. Notably, methodological factors are modifiable and, upon manipulation, alter response rates to ET, eliciting increases in CRF regardless of an individual's biological predisposition. Particularly, prescribing exercise intensity relative to a physiological threshold (e.g., ventilatory threshold) is shown to increase CRF response rates compared to when intensity is anchored relative to a maximum physiological value (e.g., maximum heart rate). It is, however, uncertain whether the increased response rates are primarily attributable to reduced response variability, greater mean changes in CRF or both. Future research is warranted to elucidate whether more homogeneous chronic adaptations manifest over time among individuals, as a result of exposure to more homogeneous exercise stimuli elicited by threshold-based practices.
Highlights
Cardiorespiratory fitness (CRF), measured as maximum oxygen uptake (V O2 max), represents aerobic capacity and integrates the functional capacity of numerous bodily systems and their ability to deliver and utilise oxygen (Hill & Lupton, 1923)
Whilst the health benefits attained from Endurance training (ET) extend beyond increases in CRF (Warburton & Bredin, 2017), this review exclusively focuses on variable changes to CRF in response to ET
As increases in CRF are associated with improved health and reduced risk of disease and all-cause mortality, understanding the factors which may influence response variability and how to minimise the incidence of non-response is important
Summary
Cardiorespiratory fitness (CRF), measured as maximum oxygen uptake (V O2 max), represents aerobic capacity and integrates the functional capacity of numerous bodily systems and their ability to deliver and utilise oxygen (Hill & Lupton, 1923). The HERITAGE study (Bouchard et al, 1999) was a seminal report highlighting the incidence of response variability at the individual level in a large heterogeneous cohort. This review covers the biological and methodological factors contributing to the response variability reported following ET (Figure 1). We appraise how biological factors, consisting of genetics, age, sex, and baseline V O2 max, and methodological factors, consisting of the type, volume and intensity of training, and the method used to prescribe intensity affect response rates to ET. Biological and methodological factors associated with the variable changes in cardiorespiratory fitness in response to endurance training. Prescribing exercise intensity relative to physiological thresholds reportedly increases cardiorespiratory fitness response rates compared to when prescribed relative to maximum physiological values. As threshold-based approaches elicit more homogeneous acute physiological responses among individuals, when repeated over time, these uniform responses may manifest as more homogeneous chronic adaptations thereby reducing response variability
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